1. Field of the Invention
The present invention relates to DC-DC converters with synchronous rectifiers, for use in, for example, switching power supply apparatuses and other suitable apparatuses.
2. Description of the Related Art
As is commonly known, DC-DC converters incorporated in switching power supply apparatuses are used to convert a DC input voltage Vin into an AC voltage in response to the switching operation of a switching element, e.g., a MOS-FET (metal oxide semiconductor field-effect transistor), rectify and smooth the AC voltage by a rectifying/smoothing circuit, and output a DC voltage Vout to a load. In these DC-DC converters, the output voltage Vout can be variably controlled by controlling the switching operation of the switching element. In other words, the ratio of the input voltage Vin to the output voltage Vout (input-output conversion ratio) is determined by the switching operation of the switching element. The output voltage Vout is detected and the switching operation of the switching element is controlled on the basis of the detected voltage so that the output voltage Vout can be regulated to a preset voltage. Recently, an increasing number of DC-DC converters using a synchronous rectifier as a rectifier in the rectifying/smoothing circuit to reduce the conduction loss have been used.
Due to a sudden increase in the input voltage Vin or a decrease in the current flowing in the load, a voltage (overshoot voltage) that is greater than the output voltage Vout, which is supplied from the DC-DC converter to the load, may be applied at the output side of the DC-DC converter.
In such a case, due to the application of the overshoot voltage, a smoothing capacitor of the rectifying/smoothing circuit is charged by the overshoot voltage. Subsequently, when the application of overshoot voltage is removed, the voltage at the output side of the DC-DC converter returns to a steady voltage. At this moment, the charge in the charged smoothing capacitor is released. In response to the application of the overshoot voltage, the switching operation of the switching element is controlled to reduce the output voltage Vout. As a result, a voltage obtained by multiplying the input voltage Vin by the input-output conversion ratio becomes smaller than the voltage across the smoothing capacitor. Thus, the charge released by the smoothing capacitor flows toward the input side of the DC-DC converter, thus generating a reverse current flowing from the output side to the input side of the DC-DC converter.
The amount of reverse current flowing becomes very large even when the overshoot voltage is only slightly higher than the steady output voltage Vout. Many problems may occur due to this large reverse current flowing.
Such a large amount of reverse current flows because the DC-DC converter with the known synchronous rectifier has regulation characteristics such as those shown in FIG. 12A. Specifically, the known DC-DC converter for rectifying/smoothing a current by the synchronous rectifier and for outputting a DC output voltage Vout has, as shown in FIG. 12A, regulation characteristics in which the amount of change in the output voltage Vout in the increasing direction (slope) is gradual compared with the amount of change in the output current in the decreasing direction, even in a reverse current flowing region. In a region where the output current shown in FIG. 12A is positive (+), the output current flows from the input side to the output side of the DC-DC converter. A region where the output current is negative (xe2x88x92) is the reverse current flowing region in which the reverse current flows from the output side to the input side of the DC-DC converter.
Since the known DC-DC converter has regulation characteristics such as those shown in FIG. 12A, for example, the application of an overshoot voltage Vx, which is very slight, at the output side of the DC-DC converter causes a high reverse current Ix to flow through the DC-DC converter.
Since this high reverse current flows, a high stress is placed on the components of the DC-DC converter, and the components may be damaged. In a DC-DC converter including a transformer, a rectifying/smoothing choke coil, and a synchronous rectifier, a large amount of electromagnetic energy generated by the reverse current flowing in a period during which the switching element is ON is accumulated in the choke coil and the transformer. When the switching element is turned OFF, a large voltage due to the accumulated energy is applied to the switching element and the rectifying/smoothing synchronous rectifier, and the switching element and the synchronous rectifier may be damaged. The components of the DC-DC converter may be damaged by the reverse current flowing.
FIG. 11 shows a parallel running configuration including a plurality of DC-DC converters (two in FIG. 11, namely, DC-DC converters A and B) connected in parallel to a load. When such parallel running is performed, the parallel-connected DC-DC converters A and B may have different output voltages Vout. In such a case, a reverse current flows from the DC-DC converter A that has a higher output voltage Vout toward the DC-DC converter B that has a lower output voltage Vout. Due to the reverse current, a circulating current is generated between the DC-DC converters A and B.
For example, the DC-DC converter A that has the higher output voltage Vout has regulation characteristics indicated by the solid line A in FIG. 12B, and the DC-DC converter B that has the lower output voltage Vout has regulation characteristics indicated by the solid line B in FIG. 12B. The parallel-running DC-DC converters A and B, as a whole, supply a current Ic to the load. In this case, a reverse current Ib based on the output voltage Vout of the DC-DC converter A flows from the DC-DC converter A to the DC-DC converter B having the lower output voltage Vout. Due to this reverse current flowing, the DC-DC converter B has a loss Ib.
In contrast, the DC-DC converter A is required to output a current Ia (Ia=Ib+Ic) in order to compensate for the loss Ib due to the reverse current flowing relative to the amount of current supplied to the load Ic. As a result, the amount of current flowing in the DC-DC converter A is increased, and the loss is increased. As described above, when parallel running is performed, if the parallel connected DC-DC converters A and B have different output values Vout, the DC-DC converter A having the higher output voltage and the DC-DC converter B having the lower output voltage both have an increased amount of loss. As a result, the circuit efficiency is deteriorated.
In order to overcome the problems described above, preferred embodiments of the present invention provide a DC-DC converter which has a synchronous rectifier and which is capable of minimizing the amount of reverse current flowing, preventing the components from being damaged by the reverse current flowing, and preventing an increase in loss due to the reverse current flowing while parallel running is performed.
According to a preferred embodiment of the present invention, a DC-DC converter for converting an input voltage at an input-output conversion ratio and for outputting the converted voltage to a load includes a switching element, the input voltage being converted in response to the switching operation of the switching element, and the input-output conversion ratio being determined by the switching operation of the switching element, a choke coil through which the converted voltage is applied to the load, a synchronous rectifier, a reverse current detector for detecting a reverse current flowing from the output side to the input side of the DC-DC converter, and a reverse current suppressor for suppressing the amount of reverse current flowing when a reverse current is detected. The choke coil has a function of increasing the inductance when the current is within a current operating range that is less than or equal to a predetermined value and decreasing the inductance when the current is in a current operating range that exceeds the predetermined value.
The reverse current detector may include a unit for detecting a control terminal (gate or base) voltage of the switching element, and a unit for directly or indirectly detecting, from the output voltage of a transformer, the voltage across the switching element (drain-to-source or collector-to-emitter). In a case in which the control terminal voltage of the switching element is at a level at which the switching element is turned off and the voltage across the switching element is low, it may be determined that the case corresponds to a reverse current state.
The choke coil may include a core which easily becomes locally magnetically saturated at one place in a magnetic path, and the choke coil may have swinging characteristics in which the inductance decreases when the amount of current flowing through the choke coil becomes a predetermined value or greater, thus causing local magnetic saturation in the core.
The choke coil may be constructed by serially connecting an inductor which has a small inductance and which is difficult to magnetically saturate and an inductor which has a large inductance and which is easily and magnetically saturable.
The reverse current suppressor may suppress the amount of reverse current flowing by controlling the input-output conversion ratio, which is determined by the switching operation of the switching element, in the increasing direction when a reverse current is detected.
The reverse current suppressor may turn off the synchronous rectifier or may bring forward the time at which the synchronous rectifier is turned off when a reverse current is detected.
According to another preferred embodiment of the present invention, a DC-DC converter for converting an input voltage at an input-output conversion ratio and for outputting the converted voltage to a load includes a switching element, the input voltage being converted in response to the switching operation of the switching element, and the input-output conversion ratio being determined by the switching operation of the switching element, an inductance element through which the converted voltage is applied to the load, a synchronous rectifier, a reverse current detector for detecting a reverse current flowing from the output side to the input side of the DC-DC converter, and a reverse current suppressor for suppressing the amount of reverse current flowing when a reverse current is detected. The reverse current suppressor includes a unit for turning off the synchronous rectifier or bringing forward the time at which the synchronous rectifier is turned off when a reverse current is detected.
The reverse current detector may include a unit for detecting a control terminal (gate or base) voltage of the switching element, and a unit for directly or indirectly detecting, from the output voltage of a transformer, the voltage across the switching element (drain-to-source or collector-to-emitter). In a case in which the control terminal voltage of the switching element is at a level at which the switching element is turned off and the voltage across the switching element is low, it may be determined that the case corresponds to a reverse current state.
According to preferred embodiments of the present invention, when the reverse current detector detects a reverse current, the reverse current suppressor controls the input-output conversion ratio, which is determined by the switching operation of the switching element, in the increasing direction, turns off the synchronous rectifier, or brings forward the time at which the synchronous rectifier is turned off.
Since the input-output conversion ratio, which is determined by the switching operation of the switching element, is controlled by the reverse current suppressor in the increasing direction, the DC-DC converter of preferred embodiments of the present invention has regulation characteristics in which the amount of change in the output voltage in the increasing direction is sudden, compared with before, relative to the amount of change in the reverse current in the increasing direction in a reverse current flowing region. In particular, by exerting the function of increasing the inductance in a current operating range in which the current in the inductance element (choke coil) is less than or equal to a predetermined value, the amount of reverse current flowing can be further minimized. Thus, for example, when an overshoot voltage is applied at the output side of the DC-DC converter, a stress placed on the components of the DC-DC converter can be greatly reduced compared with before.
Accordingly, the components are substantially protected from damage by the reverse current flowing, and the durability and reliability of the DC-DC converter are greatly improved. When parallel running is performed, if a plurality of DC-DC converters connected in parallel to each other have different output voltages, the amount of reverse current flowing due to the differences in the output voltages can be minimized. It is thus possible to absorb the increase in loss of the DC-DC converter caused by the reverse current flowing, and deterioration of circuit efficiency can be prevented.
When a reverse current is detected, the reverse current suppressor turns off the synchronous rectifier or brings forward the time at which the synchronous rectifier is turned off. In this case, the flow of reverse current can be reliably cut off by performing off operation of the synchronous rectifier.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.